1. Field of the Invention
The present invention relates to a method for flushing and calibrating at least one sensor in a system for body fluid analysis.
2. Description of the Prior Art
When a body fluid, such as blood, undergoes extracorporeal analysis by a known system for continuous or semi-continuous analyses with the aid of a venous or arterial catheter, the heart or an external pump pumps blood to a sensor outside the body. After the blood analysis performed by the sensor, blood is forced back to the body by pumping flushing liquid in the opposite direction. When most of the blood has been forced back to the patient, additional flushing liquid flushes the sensor to remove all remaining blood residue. This flushing liquid can either be infused into the patient or collected in a special bag. When this flushing is performed, the flushing liquid, which is then in contact with the sensor, is sometimes used for calibration. The sensor's characteristics nevertheless change over time. Periodic two-point calibration is commonly performed in order to attain acceptable accuracy and a reasonable operating life for the sensor.
An article entitled "Integrated pO.sub.2, PCO.sub.2, pH sensor system for on-line blood monitoring", Sensors and Actuators B, 18-19 (1994), pp. 704-708, by Gumbrecht et al. describes a blood analysis system prepared for two-point calibration. The blood analysis system has two calibration pumps, a sampling pump, two liquid containers, a collection container and a sensor arrangement. In conventional two-point calibration, as in the use of the aforementioned blood analysis system, two-point calibration is always performed in two calibration determinations using different flushing liquids between two blood analyses.
In the aforementioned blood analysis system, two-point calibration is performed at certain intervals. To ensure that the flushing liquids really have the right analytic content when intervals between calibrations are long, the tubing must be flushed, a process which is time-consuming and requires a fast pumping rate.
Instability and a shortened operating life for the system in continuous or semi-continuous blood analyses are other disadvantages of this calibration method, since frequent two-point calibration greatly increases intervals between blood analysis determinations.